I have a friend in Wisconson who keeps a large goldfish pond. I'm sure it's much deeper than 2'. She overwinters her gf outdoors. She's built a moveable shack over the pond as well as uses one of those stock heaters and probably some kind of aeration. Also, at low temps, the gf do not feed. I imagine koi could be treated similarly, though adult koi grow to 2' in length whereas gf grow to about a foot in length.
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